Session Initiation Protocol (SIP) is an application-layer control (i.e., signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet-based telephone calls, multimedia distribution, multimedia conferences, instant messaging conferences, interactive voice response (IVR), automated and manual operator services, automatic call distribution, call routing, etc. SIP invitations or INVITES may be used to create sessions and may carry session descriptions that allow participants to agree on a set of compatible media types. SIP may use proxy servers to help route requests to a user's current location, authenticate and authorize users for services, implement provider call-routing policies, and/or provide other features to users. SIP may also provide a registration function that allows users to upload their current locations for use by proxy servers.
Theft of service is the unauthorized interception and/or receipt of any communications service offered over a system without the consent of the system operator. For example, theft of service usually occurs if an individual(s) makes illegal physical connections to a system or alters (or installs) any unauthorized equipment so that the system signal can be received without the authorization or knowledge of a system operator. Theft of service currently threatens telephony services (e.g., Voice over Internet Protocol (VoIP) services) because it generates lost revenue for the system provider, and creates serious implications for the system provider in terms of legal liability, reputation, and/or user satisfaction.